There are things on my wishlist for WP-Markdown, but it works quite well.
In general, I find writing in Markdown a faster & more productive experience compared writing in the visual editor (or even with the text-only editor). The idea of duel-viewing your post (markdown & actual rendering) is useful, as you can check your work.

It would be even better to have support for a few things, like highlight.js included for writing code in blogs :)

This plugin does exactly what it says it will do, i.e. allow content editors and administrators to write content using Markdown. It works properly and has a nice preview interface - an easy 5 star review for me.